The invention relates to improvements in paper machine headboxes, and more particularly to improvements in headbox slice chambers and to an improved trailing element which extends freely toward the slice opening and is anchored at the upstream end for being self-positionable and for maintaining fine scale turbulence in the stock at the slice opening.
The present invention is an improvement in our invention disclosed and taught in our co-pending application, Ser. No. 555,158, filed Nov. 25, 1983, the contents and disclosure of which are incorporated herein by reference.
The concept of a freely movable self-positionable trailing element in a slice chamber of a headbox was first disclosed in U.S. Pat. No. 3,939,037, Hill. In a further application, U.S. Pat. No. Re. 28,269, Hill et al, trailing elements are disclosed extending from pondside to pondside. These trailing elements are capable of generating or maintaining fine scale tubulence in the paper stock flowing toward and through the slice opening. The concepts of the foregoing patents may also be employed to utilize their advantage and to function in the machine for making multi-ply paper wherein stocks of different characteristics are fed to chambers on opposite sides of the trailing elements where the elements extend from pondside to pondside.
A limitation in the headbox design utilizing the features of the foregoing patents has been that the means for generating turbulence in fiber suspension in order to disperse the fibers has been only comparative large scale devices. With such devices, it is possible to develop small scale turbulence by increasing the intensity of the turbulence generated. Thus, the turbulent energy is transferred naturally from large to small scale, and the higher the intensity, the greater the rate of energy transfer and hence, the smaller the scale of turbulence sustained. However, a detrimental effect also ensues from this high intensity large scale turbulence, namely, the large waves and free surface disturbance developed on the fourdrinier table. Thus, a general rule of headbox performance has been that the degree of dispersion and level of turbulence in the headbox discharge was closely correlated, i.e., the higher the turbulence, the better the dispersion.
In selecting a headbox design under this limiting condition then, one could choose at the extreme, either a design that produces a highly turbulent well dispersed discharge, or one that disperses a low turbulent, poor dispersed discharge. Since either a very large level of tubulence or a very low level (and consequent poor dispersion) produced defects in sheet formation on the fourdrinier machine, the art of the headbox design has consisted of making a suitable compromise between these two extremes. That is, a primary objective of the headbox design up to the time of developments of the self-positionable element has been to generate a level of turbulence which was high enough for dispersion, but low enough to avoid free surface defects during the formation period. It will be appreciated that the best compromise would be different for different types of papermaking furnishes, consistencies, fourdrinier table design, machine design, machine speed and the like. Furthermore, because these compromises always sacrifice the best possible dispersion and/or the best possible flow pattern on the fourdrinier wire, it is deemed that there is a great potential for improvement in headbox design today. While reference herein is made to formation on a fourdrinier, it will be understood that the features of the present invention and the discussed defects of the prior art also apply to twin wire formers.
The unique and novel combination of elements of the aforementioned patents provide for delivery of the stock slurry to a forming surface of a papermaking machine having a high degree of fiber dispersion with a low level of turbulence in the discharge jet. Under these conditions, a fine scale dispersion of the fibers is produced which will not deteriorate to the extent that occurs in the turbulent dispersion which is produced by conventional headbox designs. It has been found that it is the absence of large scale turbulence which precludes the gross reflocculation of the fibers since flocculation is predominately a consequence of small scale turbulence decay and the persistence of the large scale turbulence. Sustaining the dispersion in the flow on the fourdrinier wire then leads directly to improved formation.
The method by which the above is accomplished, that is, to produce fine scale turbulence without large scale eddies, is to pass the fiber suspension through a system of parallel cross-machine channels of uniform small size but large in percentage open area. Both of these conditions, uniform small channel size and large exit percentage open area, are necessary. Thus, the larger scales of turbulence developed in the channel flow have the same order of size as the depth of the individual channels by maintaining the individual channel depths small, and the resulting scale of turbulence will be small. It is necessary to have a large exit percentage open area to prevent the development of large scales of turbulence in the zone of discharge. That is, large solid areas between the channels exits would result in large scale turbulence in the wake of these areas.
In concept then, the flow channel must change from a large entrance to a small exit size. This change should occur over a substantial distance to allow time for the large scale coarse flow disturbances generated in the wake of the entrance structure to be degraded to the small scale turbulence desired. The area between channels approaches the dimension that it must have at the exit end. This concept of simultaneous convergence is an important concept of design. This concept is employed in accordance with the teachings of our previous application referred to above, and a trailing element is provided which has further improved features.
Under certain operating conditions, the trailing members which are employed to obtain the fine scale turbulence are not necessarily stable. Cross-machine transient pressures tend to bend the trailing element in a cross-machine direction and cause cross-machine uniformity variances in the paper. Resistance to deformation along the machine direction length of the trailing elements can cause slight digressions in the uniform velocity of the stock flowing off the surfaces at the trailing edge of the trailing element. Static or dynamic instability can occur at certain operating conditions and resonant frequencies can be reached dependent on the hydrodynamic forces. It has been discovered that the inertia and hydrodynamic couplings can be broken by suitable distribution of the mass and elasticity of the trailing structure with the proper mass distribution and stiffness distribution being of importance.
It is accordingly an object of the invention to provide an improved trailing element design which avoids the disadvantages that occur at certain operating conditions in structures heretofore available. It is particularly an objective to provide a trailing element which has different flexure physical qualities from the upstream end to the downstream end, and these are obtained by lamination construction. As used herein, machine direction will refer to the flow direction of the stock in flowing through the headbox, and cross-machine direction is the direction at right angles thereto. Isotropic means having the same properties in all directions, and anisotropic means not isotropic, that is, exhibiting different properties when tested along axes in different directions.
In accordance with the principles of the invention, objectives of the design are attained by providing a self-positionable trailing element which has a greater structural stiffness in a machine direction at the upstream or mounting end, and a greater structural stiffness in the cross-machine direction at the downstream or trailing end. In a preferred form, the element is made of an anisotropic material, preferably one being formed of a laminate with separate layers of the laminate providing the qualities of difference in stiffness and flexibility by either material properties, direction, size or number. Alternates of woven or needled material with weave direction or materials or size or numbers of filaments controlling directional stiffness may be used. Preferably, however, the difference is attained by the use of fibers which are arranged in a machine direction at the upstream end to provide the greater stiffness in the machine direction, and which are arranged in a cross-machine direction at the downstream end to provide for a greater stiffness in the cross-machine direction. A further feature is to provide strength at the supporting bead at the upper end which precludes the chance of adhesive failure and which eliminates the necessity of a joint to avoid cleanliness problems. A filler is added in a single wedge to prevent collapse and a cross-machine direction fiber is utilized inside to minimize cross-machine direction thermal expansion. In the downstream direction, the downstream portion of the trailing element has a dominance of cross-machine direction fibers on the outside of the sheet which maximizes cross-machine direction stiffness to reduce buckling. The dominance of cross-machine fibers on the outer surface as well as the relatively thin dimension of the trailing edge, maximizes cross-machine direction stiffness and minimizes machine direction stiffness for the tip to be able to conform to streamlines putting minimal disturbances in the flow. The thin tip with minimal machine direction stiffness and strength, yet maximized cross-machine stiffness for maximized cross-direction profile stability and minimized flow disturbance reduces eddy generation. This also allows for the use of maximum length sheets with minimum tip gap for maximum formation capability and minimum turbulence, minimum eddy generation and ability to follow streamlines and where used for a multi-ply sheet allows minimum disturbance for formation which contributes to layer purity.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: